In the process of production of chlorine and caustic, a nearly saturated NaCl brine solution is fed to an electrolyzer, where upon application of DC current, chlorine gas is evolved at the anode while water is electrochemically reduced to gaseous hydrogen and hydroxyl ions at the cathode. Anode and cathode are typically separated by a microporous diaphragm or perfluorinated cation exchange membrane such as those known under the trademarks, Nafion®, Flemion® or Aciplex®. Some chloralkali plants may still utilize the older mercury-based process, in which there is no separator between the electrodes and the cathode reaction is formation of Na-amalgam.
All chloralkali plants require a purified brine feed, and in the case of membrane plants, the brine has to be “super pure”, with the total hardness causing the metals content to be specified at less than 30 ppb. Raw NaCl brine is typically prepared from solid NaCl and water in a brine saturator. It then undergoes one or two stages of purification to remove the hardness metals, such as Ca and Mg, as well as other metallic and non-metallic impurities. Both chemical precipitation methods and ion exchange methods are commonly used to purify the NaCl brine for membrane cell plants. The brine preparation and processing stages are commonly referred to as the Brine Treatment section of the chloralkali plant and can account for up to 10-15% of the total plant cost. A detailed description of chloralkali brine preparation and purification is given, for example, in a chapter on Chlorine in Industrial Inorganic Chemicals and Products—An Ullmann's Encyclopedia, vol. 2, Wiley-VCH, Weinheim, 1999, pp. 1123-1255.
A typical concentration of the feed brine is 305±10 g/L and in the course of membrane cell electrolysis, the NaCl concentration gets depleted by approximately a third, i.e. to 190±10 g/L. Brine, after electrolysis, is called Spent Brine, Weak Brine or Return Brine. Spent Brine is first de-chlorinated and then, typically, returned to a brine saturator, as one of several make-up components to prepare the raw feed brine. It should be appreciated that Spent Brine is very pure, i.e. “super-pure”, when in the membrane plant and its recycle to the brine saturator is solely for the purpose of getting it re-concentrated back to the original strength, i.e. the 305±10 g/L.
If a convenient and inexpensive way of re-concentrating Spent Brine back to Feed Brine strength were found, then the size, and therefore the cost, of the brine treatment section could be significantly reduced. There would also be a concomitant reduction in brine treatment chemical consumption, such as NaOH, Na2CO3 and IX resin regeneration chemicals. Unfortunately, use of conventional evaporation as a means of re-concentrating Spent Brine is deemed to be prohibitively expensive, since in the typical chloralkali plant there is no extra thermal energy, e.g. steam, available. Furthermore, due to corrosivity of the brine the evaporator would have to employ expensive metallurgy.
There is, therefore, a need for improved methods of re-concentrating Spent Brine to Feed Brine.
The mechanism of water vapor transfer across a membrane in Osmotic Membrane Distillation (OMD) is based on molecular diffusion, or, in the case of smaller membrane pores, a mixed molecular and Knudsen diffusion. In either case, the rate of transfer is proportional to the water vapor difference across the membrane, membrane porosity and the reciprocals of membrane thickness and tortuosity. Suitable membrane materials include, for example, microporous fluoropolymers PTFE, FEP, PFA, PVDF, and the like, polyolefins, such as, PP, PE, and polysulfones and the like. It is also possible to use microporous inorganic materials, including carbon and glass, provided that they have been made hydrophobic by either (i) mixing with any of the above polymers or (ii) surface treatment, e.g. with organic silicones. Alternatively, it is also possible to use a thin non-porous membrane film made of polymer characterized by high free volume which makes it permeable to gases and water vapor. Examples of such polymers are poly (1-trimethylsilyl-1-propyne) (PTMSP) or 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole/tetrafluoroethylene copolymer (Teflon AF® 2400) as a water vapor permeable membrane. The non-porous membrane gives positive assurance that there will be no intermixing of the aqueous streams, albeit, at the expense of a lower flux i.e. rate of water vapor transfer normalized to the membrane area.
A good description of the OMD technique is contained in a paper by P. A. Hogan et al., “A New Option: Osmotic Distillation”, Chemical Engineering Progress, July 1998, incorporated herein as a reference. To date, OMD has been rarely used industrially, and almost exclusively for concentration of aqueous food process streams, such as juices, fermentation broths or pharmaceutical intermediates. In such applications, the user wants to avoid thermal degradation of the feed constituents, such as, for example, flavor compounds by limiting the OMD process operating temperature to the essentially ambient level. The receiver solution or “water sink” is, typically, a concentrated solution of CaCl2, MgCl2, NaCl, potassium hydrogen phosphate(s) or pyrophosphate(s). It is also possible to use low vapor pressure water-miscible organic solvents, such as ethylene glycol. In most cases, the spent i.e. diluted receiver solution is re-concentrated back to the original strength in the external evaporator. Thus, net input of thermal energy is required for the overall process. As mentioned, hereinabove, for common cases employing chloride-based receivers the evaporator would have to be made of expensive, corrosion-resistant alloys.
The phenomena of osmotic membrane distillation (OMD) has, to-date, been exploited almost exclusively for the purpose of concentration of heat-degradable food or pharmaceutical products, such as fruit juices, milk, coffee, enzymes, vitamins, and the like. The aforementioned products cannot, in general, be concentrated by conventional thermal evaporation, without negatively affecting their organoleptic or therapeutic properties.
U.S. Pat. No. 4,781,837, granted Nov. 1, 1988 to Lefebvre, Michel S. M., discloses an OMD process for concentration of fruit or vegetable juices, milk, whey, by contacting such, across a hydrophobic, microporous barrier with a highly concentrated receiver solution of salt, such as NaCl or MgSO4. U.S. Pat. No. 4,781,837 also discloses a process whereby the spent receiver solution per se is re-concentrated, e.g. by Reverse Osmosis (RO) and re-cycled back to the OMD stage. The cited process temperature is 40° C. U.S. Pat. No. 4,781,837 also discloses a concept of extracting potable water from seawater, by a combination of OMD and RO.
U.S. Pat. No. 5,098,566, granted Mar. 24, 1992 to Lefebvre, Michel S. M. discloses a hydrophobic microporous membrane with optimized thickness and porosity, particularly useful for the OMD process. U.S. Pat. No. 5,098,566 explicitly teaches the differentiation between Membrane Distillation (MD) and OMD, with the former technique not being isothermal and always conducted under a significant a temperature gradient, i.e. 50° C. or so across the hydrophobic microporous membrane.
U.S. Pat. No. 5,382,365, granted Jan. 17, 1995 to Deblay is somewhat similar and exemplifies cases of OMD-enabled concentration of liquid pharmaceutical intermediates or grape juice as well as solid food products (sliced apples). The preferable dehydrating agent (Receiver) is a concentrated solution of CaCl2. Regeneration and re-cycling of the Receiver solution is specifically described.
U.S. Pat. No. 5,824,223, granted Oct. 20, 1998 to Michaels et al. discloses use of a variety of oxy-phosphorus salts, as non-halide Receivers for application in OMD. The proposed compounds have the advantage of being highly soluble, non-toxic and non-corrosive.
U.S. Pat. No. 5,938,928, granted Aug. 17, 1999 Michaels A. N. discloses an OMD process for the concentration of juices and beverages, which utilizes a laminate membrane consisting of a hydrophobic microporous layer laminated with a thin non-porous hydrophilic film. Such laminate structure was shown to be effective in preventing wetting of the OMD membrane.
U.S. Pat. No. 6,383,386, granted May 7, 2002 to Hying et al. discloses ceramic microporous membranes coated with a hydrophobic agent in a context of membrane reactors as well as for OMD concentration of fruit juices.
U.S. Pat. No. 6,299,777 B1, granted Oct. 9, 2001 and U.S. Pat. No. 6,569,341 B2, granted May 27, 2003 to Bowser, John J. describe OMD processes which utilize non-porous, hydrophobic membranes made of high free-volume, perfluorinated polymer, such as perfluoro-2,2-dimethyl-1,3-dioxole. U.S. Pat. Nos. 6,299,777 and 6,569,341 teach that acceptable water vapor fluxes are realized with such high free-volume polymeric materials, while the materials also positively eliminate membrane wetting.
It should be noted that the prior art OMD processes operate not only at relatively low temperatures of about or below 50° C. in a non-heavy chemical environment, but also for, in effect, producing, in consequence of the OMD process, an “unwated” diluted “spent” receiver solution, which must be re-concentrated back to its original strength and recycled to the OMD process. Such additional treatment involves capital and operating costs. Thus, the prior art processes provide only a single benefit of desired product concentration, with its attendant aforesaid cost.